


A Lesson in Comparative Seduction Techniques

by weneedtotalkaboutsherlock (Paradoxe1914)



Series: The Moon and the Sea [2]
Category: Sherlock (TV)
Genre: ... slit sex, All kinds of porn just for you, Alternate Universe - Merpeople, Anal Sex, Banter, Established Relationship, Human John, Human Sex, Human Sherlock, I got creative, Interspecies Sex, M/M, Merlock, Merman John, Merman Sherlock, Merman sex, Okay there's a bit of plot though, Oral Sex, Plot What Plot/Porn Without Plot, Switching, so this means...
Language: English
Status: In-Progress
Published: 2019-04-23
Updated: 2019-04-22
Packaged: 2020-01-24 08:01:10
Rating: Explicit
Warnings: No Archive Warnings Apply
Chapters: 1
Words: 5,862
Publisher: archiveofourown.org
Story URL: https://archiveofourown.org/works/18567220
Author URL: https://archiveofourown.org/users/Paradoxe1914/pseuds/weneedtotalkaboutsherlock
Summary: John gets a lesson in seducing mermen, and Sherlock gets a lesson in seducing humans. It goes about as well as you can expect.





	A Lesson in Comparative Seduction Techniques

**Author's Note:**

> This is my first PWP! *pops a bottle of champagne*  
> Okay, there's actually plot in there too, mind you, but not a whole lot. This follows here there be monsters and your home, but you don't need to have read that to understand this. Basically, John and Sherlock are living together, in Greece, in both a human house and a merman nest. Sherlock was born a merman, and John was born human, but after the events of here there be monsters, they both spend one month in human form, and the next one in merman form. Sherlock always transforms back a few days before, which leaves a gap where they are not the same species. Also, if you're wondering, yes, Sherlock doesn't speak perfect English. It's intentional. ;)
> 
> This has a very different atmosphere from the first one. It's basically porn, humor and fluff. A human trying to find his way around merfolk sex, and a merman trying to find his way around human nonsense, with a lot of banter along the way. There are three chapters, and therefore you can expect merman/merman sex, human/human sex, and merman/human sex. They are definitely switching in this one, too. 
> 
> I should mention that the first chapter definitely has the... weirdest sex of it all. I got creative. I invented the species, so why not?! Also, I should mention that there are some A/B/O elements that have inspired me for the smut in the first chapter. It's purely physical and not at all present in the relationship's dynamics. I'm not a huge fan of A/B/O (nothing wrong with it, just not my kind of thing!), so if you aren't as well, don't worry, it shouldn't be too present. 
> 
> Enjoy!

 

John thought, after a year of living with Sherlock, that he knew everything he needed to know about the man and his species. He is about to be proven wrong.

John knows, of course, that summer is the hardest time of the year on Sherlock. He'd been quite clear about it when he had described to him the ways of his people, how mermen grew naturally more aggressive, more prone to challenge other males as mating time rolled around. But John was also convinced that having gone through some of Sherlock's bad moods and eccentricities already — which happened equally in human or merman form — that surviving three months of a hormonal peak would not be so different as usual.

Again, wrong, wrong, wrong.

He is starting to realize it now, as he is gazing at Sherlock from the corner of his eyes, watching as the merman rolls around the shallow basin of their cave, their home. They have just finished decorating, but Sherlock, as ever, is going around it again, moving every curiosity they own a few centimeters to the side, only to replace it in its original spot, his tail flicking away like an annoyed cat. John recognizes this behavior just too well: its characteristic of the few days before the new moon, which also spikes up Sherlock's hormonal cycle.

That, _plus_ summer being the mating season, turns Sherlock into what John calls privately, and not very kindly, an _annoying dick_. _Horny gay merman_ would be kinder, and something of one of those silly romances titles with shirtless men on the cover, but Sherlock doesn't even seem to be horny. Just… an annoying dick, really.

John himself is coping fairly well: even though his merman transformation is complete every full moon, he responds with less intensity to the movements of the moon and the hormonal cycle associated with it. Sure, he _does_ feel a bit cranky himself and could use a wank, but he'd rather not abandon Sherlock when the man seems so close to hysterics. Sherlock did say that some male-male couples cope better when they are apart during the mating months, but John does not want that to become a reality for them, and knows that Sherlock wouldn't stand for it either. They survived last summer, for God's sake, even though John wasn't in merman form at the time. Does that make it worse? He really can't tell.

He pops on his elbows, tired of lying around and rehashing the same thoughts over and over again. A few meters away, Sherlock is rearranging their collection of old, broken watches, water droplets glistening on the strong muscles of his bare back. From where he is, John can already see a few scales growing up Sherlock's flanks, a natural embellishment that will come to fruition on the new moon.

John's hand instinctively reaches for the pendant around his neck. For a second, he holds between his finger and thumb the small blue-black scale that was once Sherlock's. He watches as the fins at Sherlock's hips grip the rocks as he climbs a few centimeters against the wall, his precise fingers fiddling with a small array of forks and spoons. The round part of his behind, now out of the water, shines in deep shades of blue, and John can help but tug a bit at his own abdominal fins, the ones covering his slit.

"Sherlock," he says, his voice low.

"Hmmm," Sherlock says, not looking back at him.

John rolls on his front and ramps until he can let his body drop in the shallow basin, golden spots already growing on his tail. He swiftly swims to Sherlock and puts his hands on either side of his thin waist, just where his skin starts. "You've been working all morning," he says, but neither that nor the touching elicit any kind of response from Sherlock. "Honestly, love, it looks fine."

He gently pushes on Sherlock's hips to make him turn towards him, and teasingly runs his hands over Sherlock's broad chest, until his finger lock behind his neck, pulling him down to a kiss.

"John—"

"Just—"

He kisses him deeply, placating his body against Sherlock's, feeling the nice, tingly shiver running through his body, meaning that his scales are turning to gold all over his tail. But Sherlock is tense, and barely kisses back, his mind obviously somewhere else. When John rakes at Sherlock's scales to turn them black, they flick blue again, impatiently.

"John, get off." Sherlock's once is tight and authoritative.

Frowning, John lets go of him. "What is it?"

"I'm not in the mood," Sherlock says, turning his back on him and returning to his forks. There. Not _horny gay merman_ but _annoying dick_. John knows Sherlock wants it. Sherlock is gagging for it. But for some unknown reason, there's something blocking him. Has John done anything wrong?

He drops back in the water until it covers his shoulders. "Fine," he says, but it's not aggressive, if a bit disappointed.

Sherlock seems to notice that, his shoulders slight dropping. "It's not— you're just… doing it wrong."

John gapes. Sherlock had never complained about their sex life, or seemed in any way displeased about it — quite the opposite. And suddenly John is _doing it wrong_?

"I need some air— fuck, some water… You know what I mean," he grumbles, and whips his tail hard enough to propel him underwater and out of the small cave.

He swims for a good while, relishing the momentary solitude, although his mind is far from peaceful. What did Sherlock mean? He is pretty sure that he has never done anything _wrong,_ at least since the events at the lighthouse. Since then, whatever the form they take, human or Ceasg, they always have amazing, quite, quite mind-blowing sex. In John's opinion.

If things changed, they did recently. Sherlock, who initiates at least half the time, has stopped making advances on John for a few days now. But his attitude is definitely the same as it is on the week before new moons — that of a horny, impatient prick. The only different thing, if John thinks about it, is that it's summer. Their first summer together, not counting the previous year, when they had been both humans for most of it. So it's the first summer when both Sherlock _and_ John are mermen. If John isn't doing it right… does it mean that there's a usual way in which merfolk proceed?

John closes his eyes, rolling on his back as he lets the current take him in whichever direction. During the past few months, Sherlock had taught him to rely more on his instincts, saying that he was sometimes "too much of a human for his own good." Maybe he's not… animal enough for him. He chuckles at the thought.

Right. Trusting his instincts. Maybe he can try that now, it's not like he has a thousand other solutions waiting in line.

He breathes in and out a couple of times, trying to get rid of any intrusive thoughts. When he opens his eyes again, he lets his tail flick in the way it wants to take him. Slowly, he descends towards the floor of the sea, and drags his fingers through the sand, watching as it rises into small puffy clouds.

There's nothing to be seen over the white, sandy plain, and John lets his shoulder sag, happy not to be bothered by crabs, dolphins, or the occasional Undine. That's quite unusual, when he comes to think about it, that the ocean floor suddenly seems so bare.

He rolls on himself a few times, fascinated as ever by how easy it is to move underwater. How his tail perfectly responds to his thoughts, in a way he never thought to be possible when he got it at first. It's so different from moving around on his two feet, with the Earth's gravity pulling on his limp, emphasizing everything he hates about his body. Here, in Sherlock's world — which is now also his own — he feels finally free.

He twirls on himself once again, before something catches the corner of his eye: there a clam on the ground, half-covered in sand. A strange spike of curiosity makes him shiver, and he gets closer. Remembering Sherlock's extensive lessons about clam lifecycles, he can see that this one is dead. Could it be…?

He picks it up and swims to the nearest flat rock, which he uses to insert between the clam's valves. When it pops open, his curiosity is instantly rewarded: there is a small pearl lying rolling in the bottom of the greying shell.

"I have to show this to Sherlock," he mumbles to himself, strangely proud of his discovery.

He swiftly swims back to the nest, where Sherlock is working at his morning task, apparently not feeling at all remorseful about their earlier row.

"Sherlock, check this out," he says. When Sherlock finally deigns to turn and look at him, he stares for a second at what John is holding in his hands.

"It's a clam," he states, clearly unimpressed. "There are loads of clams in the sea, John."

"It's not _any_ clam," John says, and opens it for Sherlock to see the pearl inside. He extends his hand, as an offering, but Sherlock's fins flick at his hips.

"You humans, always interested in shiny things. All that glitters is not valuable."

John crosses his arms, squeezing the clam in his fist. "It's all that glitters is not _gold_ , you prick. And never mind." He flicks his tail and swims away to the small corner of the nest where he holds the old wooden chest containing his small collection of "shiny things" that Sherlock doesn't really like. They have decorated their nest together, of course, but John's fascination with marine curiosities doesn't always reach Sherlock's interest. And since Sherlock let him pick most of the house's decorations, he isn't about to complain.

He gently deposits the clam in the chest, before he falls back into the water and goes to lie at the floor of their nest, in the small curve in the sand where they used to lay a few days ago. Back when they were having amazing, satisfying _sex_. Back when he was entirely gold and glittery and Sherlock found him valuable nonetheless. From here, he can see Sherlock's deep-blue tail waving in the water, his fins delicately spread out.

John doesn't understand what he did wrong. He thought Sherlock would like the pearl. Why is it bothering him so much that Sherlock has refused his gift? It's not the first time he's liked something Sherlock wasn't interested into. It's just a bit rude, John thinks, because this was supposed to be a gift, and Sherlock _refused_ it.

He crosses his arms over his chest, and puffs out a few bubbles. If he'd listen to his instinct, as one _annoying dick_ once told him, he would leave the nest once more and go in search for something else to please his mate, but he's too proud to move a single muscle right now. He has to assert his human side by sulking down here for a while, and occasionally, glance at Sherlock's backside, somewhere above him.

 

***

 

Eventually, John leaves the nest again, searching and searching for something that Sherlock might like. Colorful pebbles, clams or algae are strictly ignored by Sherlock, and when John does the quite dangerous job of killing a lobster with a rock, Sherlock is outright offended, even though it is his favorite. For some reason, offering food is not okay.

John continues his quest for a reason he doesn't quite understand, as the days bring them closer and closer to the new moon. There are a few scales growing alongside his flanks now, and on the rare occasions Sherlock deigns to sleep, their chaste handholding makes John sweat and wish for more.

On the morning of the new moon, John, desperate, flees the nest to have a swim in the deeper sea. The current is strong, and after a while, he lets it take him in whatever direction. In the distance, he sees some kind of wooden form that he doesn't remember ever swimming by before. He urges in its direction, discovering what remains of a canoe. He is not a history expert, but it definitely looks old.

He's going through the half-decomposed planks when his eye catches on a round object lying in the sand. He grabs it instantly, his heart squeezing hard in his chest, and raises it to his face.

"To sleep, perchance to dream…" he quotes, a smile on his face. Fantastic. Sherlock will _have_ to be excited about this one.

He swiftly swims back to the nest, holding the gift behind his back. When Sherlock finally deigns to give him some attention, John can't help but notice the dark patches of black scales growing on his tail. He can't mess this up now.

"Here, for you," he says, holding up the skull towards Sherlock.

Sherlock's fins shiver with interest as he comes closer, eyes blown wide, his long fingers gently grabbing the skull. "That's a human skull," he says, as if he can't quite believe it. Of all the things Sherlock had lost when the oil spill had happened, back in Scotland, John knows that he misses the skull the most.

"I know," John says, smiling, hoping this gift is the right one.

He doesn't doubt for a long time: Sherlock lets go of the skull in favor of throwing his arms around John, pressing his mouth to his in a searing kiss. Water tumbles over his head and John closes his eyes, barely aware that they are both sinking under from their combined weight, but neither of them cares about that.

He can't help but smile under Sherlock's lips, victorious and smug. He knows he's won his mate's good favor, and the fins at Sherlock's hips, already clutching John's waist, are the direct confirmation of what is about to happen.

He groans into the kiss, whipping the end of his tail to propel them both forward, until Sherlock's back hits the rocky wall of the cave. He placates his front against Sherlock's, feeling how their scales rub against each other, as he gently moves up and down Sherlock's body. It's like the rasp of stubble, and every single one of John's nerve is on fire. Before his first transformation, he already knew that a merman's scales were a crucial part of their mating process, but he could have never imagined how good it felt to see his body turn from green to gold, to witness his mate equally aroused, to have their scales come into contact, Sherlock's fingers gently pressing against John's pendant. If he had to qualify it, he would say it felt like an impossibly good scalp massage, the kind that sends you shivers down your spine.

He nips at Sherlock's full bottom lip, before sliding down to press his mouth to his neck, just under his jaw. Sherlock groans and John chuckles, bubbles running from his nose and tickling Sherlock's ear, who squirms under this merfolk version of blowing a raspberry.

John kisses Sherlock's jaw, watching his bottom lip trembling as he exhales a sigh. Sherlock's hands are on his waist, and they gently push him away from his own body.

"Show me," he says, looking down between them. "I want to see."

John can't help but grin as he lets one hand fall from Sherlock's neck, and travels it down his chest, until he reaches the abdominal fins covering the precise area Sherlock is staring at. He runs a finger once between the two long fins, down and back up, where he spreads them by opening his index and middle finger in a V, revealing the fine line of his slit. Sherlock's hands pressing harder in the skin at John's waist, the few bubbles gliding up between them a clear sign that he is breathing heavily.

The fat head of John's cock pushes against the top of his slit, pressuring his inward folds to show two thin stripes of pinkish skin on each side, before it pops out in an audible _plop_.

He glimpses at the hint of tongue wetting Sherlock's lips as he looks down with intense fascination at the sight of John's cock. His fins instinctually open on both sides, flashing his own swollen, pulsing slit. God, is this how mating on a new moon during the summer feels like? They have a very satisfying sex life but it's nothing compared to this, to the _urge_ of mating that flares John's every nerve. If he were just a bit less in control, he would placate his body against Sherlock and plunge his cock into that lovely, swollen opening. He logically _knows_ that they can't procreate, the both of them being male, but his summer hormones seem to have put a stop to any rational thought that isn't pushing him to hold his lover still as he works his cock inside him, pumping and pumping until he spills and fills him with his seed.

John groans, relieving some of his arousal by thrusting his cock in and out, the fat head grinding against the bit of skin where the lips of his slit meet.

"John," Sherlock whispers, pulling him closer, clearly angling their bodies so that they can join. But when John's chest finally touches Sherlock's, his cock is already retracted, leaving Sherlock whimpering, his hips drawing circles against John's skin, seeking contact with what he desires most. "There is no need in delaying—"

"Hush," John says, kissing him one last time straight on the mouth. He's been waiting for this for days now, and even though his body trembles with want, his most rational thoughts know that this will be better if he makes it last.

He dips his head and presses his mouth to Sherlock's neck again, going lower and lower until his tongue traces the outline of Sherlock's first gill.

"John!" Sherlock squirms, his hands grabbing at John's backside, fingers digging through his scales. He's always been wonderfully sensitive there, more so than John, whose body still felt more human than merman in some ways. "Again," Sherlock orders him, and John complies, licking at the small, delicate scar-like gills.

"No need in delaying, uh?" Impatient git. He moves down Sherlock's body, both of his hands on Sherlock's waist to keep his back placated against the rocky wall.

He teases him endlessly, pressing kisses into his skin, his ribs, his belly, playing with a nipple, going down again until he reaches Sherlock's scales and licks a broad dark stripe up, watching as they become blue and revert to black in a shiver. Sherlock's abdominal fin, seemingly moving as the water allows it, caresses the side of John's face, reminding him of more urgent matters.

He looks up, and see Sherlock staring down at him, his hands trying to find purchase on the rocks behind him.

"Show me." John is the one to say it, this time. He can see a faint blush making its way unto Sherlock's face, but his attention quickly turns to Sherlock's fins, who, once again, unabashedly reveal his slit.

"Going to make you all wet and ready for me," he groans, his face oh-so-close. He can feel a soft current disturbing the water around them, created by the squirming of Sherlock's tail.

He grabs him harder by the hips, makes his tongue flat, and licks one broad stripe up Sherlock's opening.

" _John!_ "

One of Sherlock's hand fly to John's head, pressing him harder against his flesh. John complies, driving his tongue inwards, pressing into Sherlock's yielding folds until he can get a proper taste of him. He smells divine and is already incredibly wet — certainly the result of those summer hormones running in his veins — and although his slick instantly dissolves into the water, John can't help but chase the taste of it by plunging his tongue harder and deeper into him.

Sherlock might be a moaning mess already, but John is quite surprised that he hasn't shown off his cock already. It usually happens pretty early on every time John goes down on him — inevitably, his mouth will be headbutted by the head of Sherlock's cock, too curious and too aroused to stay obediently tucked inside, behind the membrane that separates his cock from his… well, from the interior channel that leads to his slit. Clearly, Sherlock has decided who will be in charge of the fucking today, and John could not be happier to comply. Sherlock is both the hardest and the easiest man to please John has ever had the chance to witness.

He climbs back up Sherlock's body, licking into his mouth. "What do you want?"

"You know what I want." God, he can hear the despair in Sherlock's voice.

"Tell me."

Sherlock shakes his head, tilting his chin forward to look down.

"You want this?" John says, tilting his hips forward, letting his cock peek out of his slit.

Sherlock nods, wrapping his arms around John's shoulders.

"Tell me."

Sherlock groans, his nose in the crook of John's neck. "I want you to— penetrate me with your p— penis."

John bites on his lip, stifling a laugh. He _knew_ he shouldn't have given Sherlock that one medical textbook, but the deed is done. It's not like he would be better at dirty talk in a secondary language anyway.

"John, I swear if you're not going to put that in me right now, I'm going to bite you."

John laughs, taking Sherlock's head between his hands and kisses him one last time. "All right, all right, you feisty little thing."

He looks down, gently guiding his hips against Sherlock's, coming flush against his body. He moves his hips in tiny circles, trying to find Sherlock's slit, and gasp the second he feels Sherlock's folds grinding against his, their abdominal fins tangling together on each side.

Sherlock lets his head fall back against the rocks, lost for words. " _John_." This time, his tone isn't snarky but despairing.

"Feel that… right there, uh? Don't move," John orders, stabilizing Sherlock against the rock with one hand at his waist.

As gently as he can, he pushes his cock forward, moving a centimeter down Sherlock's body in order to get the angle right.

"Don't move, don't move," he pants, and Sherlock anchors himself to him by gripping his shoulders and back.

Steadily, he sinks his cock into Sherlock's opening, centimeter by centimeter, feeling Sherlock's walls progressively relaxing around him. They both groan when he's in all the way, and it's only then that John realizes that Sherlock has tangled their tails together, something that is instinctual to merfolk — it's certainly not the first time they've done so.

Gently, Sherlock pushes them away from the wall — their tails entangled, they can't swim that way, and so, they slowly start to sink further and further towards the sea's floor. Mating, for that reason, John understands, can be quite dangerous if not done in a nest, safe from the predators of the open sea. Which also means that the usual affair only lasts a few short minutes for the sake of safety and efficiency.

_Not_ on John's watch, though. Sherlock is one lucky sod.

The other issue with mating this way is that it's quite easy for John to accidentally slip out (which Sherlock _never_ likes), and so, their biggest challenge is to remain as motionless as possible. Which John doesn't mind, not really, because between the two of them, things never feel _motionless_.

"Are you— good?"

"John," Sherlock lets out, his voice a bit menacing, which makes John smile.

He grabs Sherlock's back, and starts to thrust his cock up Sherlock's slit, in infinitesimal movements. It takes him a minute or two to speed up, pushing harder and deeper, until he feels the swelling bump of Sherlock's own erection, hidden behind the membrane that separates them, exquisitely tighten the space around his cock.

"You're so hard, love," John pants, the tip of his prick catching unto the underside of Sherlock's cock head.

" _Mmmh_." Sherlock rasps his teeth alongside John's jaw, his fingers digging into John's back.

Well, looks like he's going to do all the work today. Not that he particularly minds. He turns his head to escape the grip of Sherlock's soft bite, and kisses him, kicking his hips forward (stupid human habit), trying to change the angle of his thrusts to hit that spot that makes Sherlock melt every time.

He knows he's close when Sherlock's body tenses with apprehension, tiny whimpers escaping his reddened lips. "Oh— there— do it, John, do it!"

He knows he has found it when he feels the two tiny almond-shaped bumps catching unto the top of his shaft, Sherlock's body shuddering all around him. He has never quite known what's the gland's function, really, apart from the fact that it acts a bit like a prostate during sex, and procures deep pleasure to his partner. John has it too, when he's in merman form, but it's a bit less sensitive than Sherlock's — or maybe the git is just being dramatic.

"There," Sherlock purrs (as if John didn't know!), every muscle in his body going slack, and John has to grip him harder in order to stay linked together. It's about the same time that he registers the ground under them, and gently cradles Sherlock's head as his back comes in contact with the sand.

Stabilized against the sea floor, John undoes his grasp on Sherlock's waist and takes his hands instead, guiding them above Sherlock's head where he pins them into the sand. This way, he has better leverage and can actually move his hips without fearing slipping out. He kisses Sherlock one last time, and thrusts forward.

Sherlock gasps, and John can feel him growing bigger and bigger against him, _inside_ of him. He fucks him in sharp, hard thrusts, his gaze lost in Sherlock's round, blue eyes, his hair a dark halo around his face, and John can already feel arousal pooling low in his belly. They're not kissing, both of their mouths open and panting swarms of bubbles. Sherlock's body rocks a centimeter forward with each thrust, raising small clouds of sand around them.

It takes John a moment to understand what is happening, as Sherlock's eyes grow dark, because he has never witnessed this before: after a moment, he can definitely feel the two almond-shaped glands _expanding_ , elongating as they start encircling the top his cock, just under the head. He keeps on thrusting forward, the friction created by it too good to stop, before he realizes he can't move as well as before, stuck between… that, and Sherlock's twitching erection. It feels incredible, but his human mind cannot help but think that something is wrong.

Under him, Sherlock looks utterly blissed out, his eyes closed, his chin propped back. He's close, John knows, and he wants to get him there, but the glands are closing off around the top of his cock and—

Panic rises in his chest, almost as strong as his current arousal. " _Sherlock!_ "

He jams his hips forwards, and the glands finish tightening completely around him, making him unable to move. Sherlock comes with a shout, and, in rhythm to the pulsing of his cock, the glands squeeze three times in quick succession around John, sending him over the edge as well.

He's never come this hard before, his orgasm somehow prolonged by the glands around him, milking every last bit of come he can offer. Head thrown back, the only thing he is aware of is Sherlock's fingers biting into the back of his hands, one last anchor to the ocean floor.

" _Sherlock_ ," he lets out again, panting and surprised.

He just came, yet his cock isn't softening, and the low heat in his belly persists. He props himself on his elbows, trying to part from Sherlock's body, but the glands are still tugging on his cock — he lets out a yelp of pain, and Sherlock's hands fly to his back to hug him close once again.

" _Don't_ move, John." Sherlock's fingers are drawing patterns over John's back, probably trying to soothe him.

"Fuck— what's that? Sherlock, what's happening?" He sounds panicky. Why does he sound panicky?

"Relax," Sherlock says. "I think— I think I'm trying to get everything."

On those last words, the glands squeeze again, three quick pulses, and John gasps, his cock spurting once more. It's not entirely unpleasant — it definitely feels like getting a second orgasm albeit a much tamer one than the first. Sherlock moans and throws his head back, sand mixing into his hair.

When he sees straight again, John asks, "Are _you_ doing that?"

"Visibly. Although not _intentionally_."

"Why?"

"I believe— oh, _oh_ , John!" It happens again, and John softly bites into the top of Sherlock's shoulder, his instincts asking him to keep his rather energetic lover still. It still comes as a surprise, but this time, John rides the wave rather enjoyably, his hands fisting into Sherlock's hair until his mouth start producing sounds that make sense again. "I… believe that I'm somehow… trying to produce offspring with you."

John jerks his head back. " _What_? But you _can't_ do that." They've made it clear that Sherlock is male, and that his anatomy is unsuited for this kind of task. For a second, John imagines their nest swarming with little baby Ceasgs, and feels dizzy. "You specifically said that you _can't_ do that."

"Calm down, John, if you move too much you're going to— argh." John stills, not particularly keen on being castrated today. Well, this is one situation when having a row with your mate can get frustrating. "I know I can't have offspring, John, I'm merely trying to make you understand that my body _doesn't_ know."

"That doesn't make any sense."

"Let me tell you in ways your silly little human brain _can_ understand, then. The Ceasgs branch evolved from an all-female species. I have already told you about the existence of pips, which are somewhere between male and female. Which means, as my brother has told me, that males have evolved _from_ pips. My… pliers—"

John winces. "Let's call them glands."

" _Fine_. My glands seem to be vestigial organs from a female's body. This," he says, waving a hand towards their joined bodies, "is clearly the first time it has happened, so I would theorize that it only does so on a black moon during summer, when Ceasgs are known to reproduce. Therefore my body is _trying_ to have your offspring, although I rationally know it cannot achieve that. Does it make sense _now_?"

John groans, somehow relieved. He remembers Sherlock telling him how reproduction during the summer has better chances when happening on new moons. Even though he didn't seem to know why at the time, that must be the answer. Bit weird to find female body parts that don't serve anymore in a male's body, but the outcome is quite pleasurable in the end. "It does. You really didn't know about this?"

Sherlock pouts. "How would I?"

"I don't know. Didn't your mothers tell you about this, or something?"

Sherlock gapes. "No! Did _your_ mother explained to you how penetrate a man's behind?"

John giggles. Well, certainly not a _man_ 's behind, of all things. "Fair enough. Can you let me go, now?"

"Nope," Sherlock says, "and I'm pretty sure of that."

"Do you know how long it—"

"John, is it enough if I say that we're in unexplored territory, and that I don't—"

John rolls his eyes and kisses Sherlock, mostly to make him shut up. It truly isn't a new moon if the man isn't intolerable at best. But God does he _love_ him. Even then.

They spend the rest of the following minutes kissing and petting each other, stopping to gasp and cling to each other as two other orgasms rip through their bodies. Finally, John feels the grip soften on him, and can't help but sigh with relief. He came five times in a short amount of time, and he shouldn't expect to be able to move an inch of his body for a long while.

Except that a long while never last with Sherlock, who climbs over him. After a minute of staring, his chin on John's chest, he orders, "my _hair_ , John."

"Oh God," he says with a chuckle. "I've just filled you with the biggest amount of come I've ever been able to produce under half-an-hour and now you want _me_ to take care of _you_?"

"Yes. I believe it's traditional, since my body is currently growing your offspring."

"Your body is growing nothing at all, you git."

" _John_ ," Sherlock moans, his mouth shaping into the most dramatic pout John has ever witnessed, his index finger running along John's clavicle.

He groans, but breathes in the water and inflates his stomach — with a few quick, lazy whips of his tail, they slowly start ascending. Sherlock smiles and nuzzles him until their bodies break into the fresh air of their nest.

Visibly quite happy with himself, Sherlock dips forward to kiss John, before he offers him, no too subtly, his head. With a sigh, John starts picking through the strands of hair, in the usual post-coital cleaning ritual Ceasgs exhibit. He doesn't understand much about it, but it's important to Sherlock, so there.

"John?"

"Mmh?"

"You… did it right," he mumbles into his shoulder.

John frowns. "What was that?"

"Nothing. I can feel your semen so _deep_ in me."

He chuckles. "And only _now_ you're indulging into a bit of dirty talk. But God, that was quite intense, wasn't it? I can feel you leaking on me, actually." Sherlock gasps, his abdominal fins falling shut, propping himself up against John's chest, who's already pulling him back towards him. "Stop worrying. No babies, remember?"

The corner of Sherlock's mouth tugs in a displeased way, before he settles back against John's chest. It's so strange, this sudden urge for Sherlock to procreate, when he's a male who specifically _cannot_. Are all males the same? John doesn't feel it, but maybe it's because his brain is too human for him to get this particular instinct. He doubts that males who do procreate feel the need to be… well, fucked, though. Is it a gay thing only, then? It might be — it's Sherlock first summer with a merman mate, after all, he was still fully human last year. But then, what distinguishes who tops or bottoms? It's not like Sherlock is more feminine than him, or ever expressed maternal desires — God, he can't imagine that. But they have both enjoyed different roles over the last year, and John knows Sherlock likes switching without much discrimination. Maybe he was just in a bit of a mood for a good fuck.

He guesses he'll have to ask him later. For now, he deserves a bit of rest and cuddling.

"John?"

"Mmh?"

"When do you think you will be able to go again?"

"Oh my _God_."

 

**Author's Note:**

> Sherlock should have gotten his Arthur voice and said, "behold! Surprising sex!" ;)


End file.
